A comprehensive study of two molybdenum-containing hydroxylases is proposed, with the goal of fully characterizing them from a physical and kinetic standpoint. Arsenite oxidase is responsible for the first step in the biodegradation of arsenite in the environment, its oxidation to arsenate. As such, the enzyme plays a critical role in coping with the toxicity of this widely distributed compound. The enzyme has recently been isolated from the soil bacterium Alcaligenes faecalis, and is found to be a molybdenum-iron/sulfur protein of molecular weight 92,000. It is the first protein in a linear electron transfer system responsible for the oxidation of arsenite, with electrons passing from the oxidase to a Type I copper protein, then to a c-type cytochrome and finally to the cytochrome oxidase of the organism. In addition to studies probing the mechanism whereby arsenite is hydroxylated to aresenate, the interactions of the various components of this multicomponent electron transport system will be investigated and its significance in the metabolism of environmental arsenicals assessed. Sulfite oxidase is a molybdoheme enzyme isolated from liver of a variety of vertebrates. It is responsible for the oxidation of sulfite to sulfate, the final step in sulfur catabolism prior to urinary excretion as sulfate in man. The crucial metabolic role of this enzyme is indicated by the serious health problems exhibited by individuals with a genetic deficiency of the enzyme, including congential brain damage and severe mental retardation. Despite a substantial amount of information on the physicochemical properties of the enzyme, the kinetic and mechanistic behavior of the enzyme has only been poorly characterized. A sound understanding of the kinetics and mechanism of sulfite oxidase is highly desirable, and to this end the proposed work is to be undertaken.